The present invention relates to an improved continuous process for the preparation of mononitro-aromatic compounds by nitrating aromatic compounds by means of aqueous nitric acid without the use of sulphuric acid.
Most nitrations of aromatic compounds are carried out in a mixture of nitric acid and sulphuric acid, the nitric acid being employed only in a small molar excess.
A known process for the preparation of mono-, di- and tri-nitrotoluene is the so-called Schmid-Meissner process (Chem. Eng. 9, 161 (1966)), in which the mixed acid nitration and working up are carried out in a continuous process. With this process, the sulphuric acid, which is diluted by the water of reaction and in some cases by water which has been additionally added, must be re-concentrated before it is reused and this requires high temperatures. A number of technical, economic and ecological problems result from this.
The nitration of aromatic compounds with nitric acid on its own in general requires a molar excess of acid relative to the aromatic compounds employed, since the concentration of nitronium ions in pure nitric acid is lower than that in mixed acid.
The water of reaction formed during the nitration leads to dilution of the nitric acid, so that the nitration cycle cannot be repeated an arbitrary number of times re-using the nitric acid which has already been employed for nitration since, as the dilution increases, the rate of the nitration reaction decreases to an ever greater extent. A process of this type can therefore not be used for continuous operation of the reaction, prerequisites for which are constant reaction conditions and above all a constant rate of reaction.
In order to ensure economical operation of the process, it was necessary to develop a process for removing the water of reaction.
Thus, a process in which the nitration is carried out in a distillation column is described in Ind. Eng. Chem. 34, 286 (1942) and 36, 447 (1944). In this case, the unreacted hydrocarbon passes over the top of the column as a water/aromatic compound mixture which boils as an azeotrope.
After separating off the water, the aromatic compound is fed back to the column as reflux. The sump product (nitro-aromatic compound and nitric acid) is separated in a second column.
Furthermore, a process for the nitration of benzene, toluene and xylene in which the nitration is carried out in excess aqueous nitric acid at 100.degree. to 120.degree. C is described in U.S. Patent Specification No. 2,739,174. By withdrawing the reaction mixture below the reaction zone, a hydrocarbon-free mixture of water, nitric acid and the nitro-aromatic compound is obtained and this is fed as such to a distillation stage. An azeotropic mixture of water and the nitro-aromatic compound is taken off over the top and is separated after cooling. The nitric acid collects in the sump of the column and is removed from there.
Rectification is incomplete with both processes, as described on page 3 of German Offenlegungsschrift (German Published Specification No.) 2,240,048, and this manifests itself in the relatively low yields. With this process it is possible to achieve an increase in the yield only with high expenditure on technical measures.
Moreover, with both processes distillation of the reaction mixture involves greater exposure to heat and thus give rise to hazards from the safety point of view (compare Chem. Eng. 9, 163 (1966)).
Furthermore, a process for the preparation of 1-nitronaphthalene by nitration of naphthalene in at most 30% strength by weight nitric acid is known from German Patent Specification No. 558,468; in this process the nitric acid which remains after the nitration product has been separated off is restored to its original content with fresh concentrated nitric acid and re-used. According to the data given in this patent specification, the reaction proceeds too violently and leads to the formation of undesired by-products when the concentration is higher than 30% by weight. The disadvantage of this process is, on the one hand, that the reaction times are long (8 hours), which makes the use of continuously operating reactors uneconomical, and, on the other hand, that the reaction volume continuously increases as fresh concentrated nitric acid is added and this stands in the way of economical cycling of the nitric acid.
With the process described in German Offenlegungsschrift (German Published Specification) No. 2,240,048 for the nitration of aromatic hydrocarbons with aqueous nitric acid, the aromatic hydrocarbon is allowed to bubble, in the vapour state, through aqueous nitric acid.
In this case, the water of reaction which is formed goes into the vapour phase as a hetero-azeotrope with the unconverted aromatic hydrocarbon and is withdrawn from the system after condensation of the azeotrope and phase separation of the organic phase.
The process has the disadvantage that relatively large amounts of the aromatic compound are fed in the cycle and the energy costs are thus necessarily higher than when the aromatic compound is largely or completely converted. Since the reaction can proceed only at the boundary layer between the gaseous and the liquid phases, the space/time yield is, moreover, smaller than in the case of a reaction which proceeds in the liquid phase.
Furthermore, a process for the nitration of halogenated benzene derivatives in which the hydrocarbon is reacted in the vapour phase with nitric acid in the presence of a solid inorganic oxide as the catalyst is described in German Offenlegungsschrift (German Published Specification) No. 2,510,095. Disadvantages of this process are the catalyst necessary for the reaction and the fact that the space/time yield is lower than that for a reaction carried out in the condensed phase.
The nitration of aromatic hydrocarbons which is described in German Offenlegungsschrift (German Published Specification) No. 2,249,373 is also subject to the same disadvantage, that is to say the necessity for a catalyst (perfluoroalkanesulphonic acids).
In another process (German Offenlegungsschrift (German Published Specification) No. 2,220,377) the nitration of anthraquinone is carried out in excess nitric acid. In this case, the nitric acid content of the reaciton mixture is higher than that of the azeotropic mixture after the nitration has ended. The reaction mixture is subsequently separated into the nitration product and two nitric acid fractions (acid content &gt; 70% by weight) of different concentrations and of these the more concentrated fraction is re-used for the nitration and the more dilute fraction serves to regulate the rate of reaction. However, this process has the disadvantage that the water of reaction cannot be withdrawn from the system as water but only in the form of dilute nitric acid and this leads to losses of nitric acid. Furthermore, aromatic compounds which are nitrated easily cannot be nitrated by this process since the reaction frequently proceeds too vigorously and in an uncontrolled manner.